Based upon the success obtained in prior research in developing glass-ceramic bodies exhibiting high strength and toughness wherein the predominant crystal phase is a relatively low-melting chain silicate (canasite in U.S. Pat. No. 4,386,162 and potassium fluorrichterite in U.S. Pat. No. 4,467,039), research was initiated to determine whether glass-ceramic bodies could be devised containing the chain silicate enstatite (MgO.SiO.sub.2) as the predominant crystal phase. Enstatite is significantly more refractory than canasite and the richterites, having a melting point in excess of 1500.degree. C.
Not only is enstatite a chain silicate having a capability for interlocking acicular microstructural development, but also it demonstrates martensitic-type transformations, viz., protenstatite--orthoenstatite upon cooling below 1000.degree. C. and orthoenstatite--clinoenstatite, when subjected to shear stress. ("Experimental Study on the Polymorphism of Enstatite", Am. Mineral, 59, 345 1974). These transformations are known to develop such microstructural laminations as twinning and exsolution which can retard fracture and increase toughness. Furthermore, the stress-sensitive coversion to the monoclinic polymorph has been shown to induce plastic flow at temperatures as low as 25 C. in compression tests under confining pressure. ("Experimental Deformation of Enstatite and Accompanying Inversion to Clinoenstatite", F. J. Turner, H. Heard, and D. T. Griggs, Int. Geol. Cong. Rep. Sess. Norden 21st. pr. 18, 399, 1960).
Glass-ceramics are produced through the controlled crystallization of precursor glass bodies. Hence, glass-ceramics are commonly prepared by means of the following three general steps: (1) a glass-forming batch, customarily containing a nucleating agent, is melted; (2) the melt is cooled below the transformation range thereof and simultaneously a glass body of a desired configuration shaped therefrom; and (3) the glass shape is exposed to a predetermined heat treatment to cause the growth of crystals in situ. Frequently, to secure a more uniform crystallization in the final product, the heat treatment will be conducted in two steps: first, the glass shape will be heated to a temperature within or somewhat above the transformation range for a period of time sufficient to develop nuclei therein; and, second, the temperature will be raised to approach and often exceed the softening point of the glass to effect the growth of crystals on the nuclei. (The transformation range has been defined as that temperature at which a liquid melt becomes an amorphous solid; that temperature generally being deemed to reside in the vicinity of the annealing point of the glass.)
Glass-ceramics are customarily highly crystalline, i.e., at least 50% by volume crystalline and, frequently, approaching 100%. Because the crystals are grown in situ on previously-developed nuclei, glass-ceramics are non-porous and free of voids, and the crystals are relatively uniformly-sized and homongenously dispersed in a residual glassy matrix. Glass-ceramics demonstrate physical and chemical properties more closely akin to those of the crystal phase than to those of the parent glass due to the high percentage of crystallinity therein. Finally, the composition of the residual glassy matrix will be different from that of the original glass since the components of the crystal phase will have been removed therefrom.
U.S. Pat. No. 2,920,971, the basic patent in the field of glass-ceramics, provides an extensive discussion with respect to the mechanisms involved in the production of glass-ceramics and microstructures that can be present in glass-ceramics. Reference is made to that patent for further understanding of the manufacture and character of glass-ceramics.
The primary objective of the present invention was to develop very highly crystalline glass-ceramic articles wherein enstatite constitutes the predominant crystal phase; the enstatite crystals imparting high refractoriness to the articles and acting to control fracture propagation, thereby enhancing the toughness of the articles.